To make the object of the invention more readily comprehensible, firstly the anatomical structure of a normal human shoulder will be described with reference to FIG. 1.
FIG. 1 shows a normal human shoulder which, in per se known manner, comprises a shoulder blade 11 and a humerus, whereof it is possible to see the upper part 16 and the humeral head 22. The substantially spherical humeral head 22 is in contact with the socket 2 of shoulder blade 11, which is a cartilaginous cavity against which slides and rolls said humeral head 22. The special feature of such a joint is that humerus 16 is held in place against shoulder blade 11 solely by passive suspension with the aid of ligaments 23a,b and active suspension with the aid of periarticular muscles connecting humerus 16 and shoulder blade 11.
In the case of acute or chronic degenerative lesions of the shoulder or fractures of the head, the humeral neck or the scapular socket, it is sometimes necessary to replace the affected or fractured parts by prosthetic implants on the humerus and shoulder blade.
Different prosthetic replacements of the shoulder have already been proposed.
The first, more particularly proposed following fractures to the humeral head prevent any assistance by osteosynthesis (screws, plates, pins), consist of a humeral prosthetic member, either sealed or not sealed with cement in the diaphyseal shaft of the humerus following the exeresis of the fractured fragments of the humeral head.
Other prostheses were then proposed and were directed towards a complete prosthetic implant, i.e. the replacement of the humeral head associated with the replacement, after exeresis, of the glenoid articular cartilage (shoulder blade) in contact with said prosthetic head.
These different solutions suffer from numerous disadvantages.
Thus, even though the positioning of the glenoid member causes no major problems, the orientation in rotation of the humeral member is difficult, due to the operating conditions and the lack of a reference point for the surgeon to establish proper spacial orientation; following the resection of the humeral head.
Moreover, the clinical results are often impaired by the fact that the condition of the muscles is often not very good prior to the operation, e.g. in elderly patients, whose musculature is not well developed in the case of a fracture, or a shoulder which has been locked for several years in the case of degenerative ailments. Furthermore, the muscular tissue is also impaired by the actual operation, which makes it necessary to cut through certain muscles in order to obtain access to the joint.
Finally, and more particularly, when there is no mechanical connection between the individual prosthetic members, either a swinging shoulder, or a very significant reduction in movements due to the reduction or absence of the synergy of the motor muscles of said prosthesized joint results.
Thus, in a normal shoulder, the stability of the humeral head is ensured, as has been seen relative to FIG. 1, by ligaments, the articular capsule and the muscles of the rotary cuff.
The ailment or disease in question, which may or may not be traumatic, leads to a deterioration of certain muscles and this is further impaired by surgery and there is a risk of injury to the ligaments and the articular capsule, through which it is necessary to pass and then repair.
It is for this reason that a second type of shoulder prostheses, illustrated e.g. by U.S. Pat. Nos. 3,978,528 and 4,206,517 have been proposed, in which all efforts have been directed at producing an artificial swivel jointed shoulder unlinked with the normal shoulder, while making no effort to restore the anatomical shoulder. In such prostheses a spherical head fixed to the shoulder blade swivels in a spherical hollow recess or cupula fixed to the humerus and takes the place of the old head, using a diaphyseal projection cemented into the humerus axis.
The designers of such prostheses correctly thought that a swivel joint is mechanically more satisfactory than a rolling contact and were clearly inspired, as can be gathered from U.S. Pat. No. 4,206,517, by the hip joint (cf. FIGS. 13, 14 and 15). However, on the one hand the operating conditions of the shoulder (generally in traction) are very different from those of the hip (generally in compression) and, on the other hand, the possibilities of dislocations of the prostheses described in U.S. Pat. Nos. 3,978,528 and 4,206,517 are very real, if the system of ligaments and support muscles is in any way inadequate.
Thus, in the two aforementioned patents, the diaphyseal projection forms an angle of approximately 60.degree. with the free surface of the cupula, which makes it necessary to connect said projection in the vicinity of the apex of the hemisphere constituting the cupula. It is also necessary to provide an intermediate mechanical system (catching plastic washer 38 in U.S. Pat. No. 3,978,528 and the association of a rigid box 14 and an elastic washer 15 in U.S. Pat. No. 4,206,517) for locking the sphere in its recess and making the cupula retentive, particularly when the arm is lowered. In other words, the risk of shoulder dislocation is only obviated through the presence of a necessarily fragile and degradable, intermediate system.
Finally, as a result of the fact that in these constructions the axis of the diaphyseal projection--or the humerus--does not pass through the centre of the swivelling sphere produces a complementary torque of the traction force of the arm with respect to the centre of the swivel joint, which makes such designs even more delicate.
All the aforementioned disadvantages have meant that hitherto no such swivel jointed shoulder prostheses have been fitted and they have remained at the planning stage.
The object of the total prosthesis according to the invention is to obviate the aforementioned disadvantages and its special construction, mechanically suspended on the shoulder blade, makes it possible to obviate any risk of dislocations, even in the case of a serious failure of the conventional support muscle and/or ligament systems of the normal shoulder.